1. Field of the Invention
The present invention relates generally to the treatment of gases containing sulfur dioxide, and more particularly, to an improvement in those processes for removal of sulfur dioxide from flue gases which use an aqueous absorbent containing sodium, potassium, or ammonium carbonate to convert the sulfur dioxide to sulfite. The term "sulfite" as used herein refers to both the sulfite and bisulfite form of the anion; and the term "carbonate" likewise refers to both the carbonate and bicarbonate form of the anion.
2. Description of the Prior Art
The art is extensive on the removal of SO.sub.2 from gaseous mixtures by means of aqueous absorbents containing alkaline sulfite-forming additives. The sulfite-forming additives include sodium, potassium and ammonium carbonates. The absorption of SO.sub.2 by suitable contact of an SO.sub.2 -containing gas phase with an aqueous solution containing such additives is both rapid and efficient, provided certain conditions are satisfied. Efficient absorption depends upon operating with scrubbing solutions which contain a low equilibrium vapor pressure of SO.sub.2 over them. Such a condition is reached when the concentration of sulfite is relatively low, i.e. less than about five weight percent, and when the pH is relatively high, i.e. greater than 6.0.
Rapid absorption can be achieved, in general, by operating with scrubbers of proper design which provide adequate contact area between the liquid and the gas and in which turbulent flow conditions are maintained.
The state of the art of sulfur dioxide removal from industrial waste gases as of the year 1958 is presented by the Bureau of Mines (U.S. Department of the Interior) in its Information Circular 7836 entitled "Sulfur Dioxide -- Its Chemistry and Removal from Industrial Waste Gases" by D. Bienstock, L. W. Brunn, E. M. Murphy and H. E. Benson.
The difficulties associated with the use of the above-cited absorbents in the treatment of flue gases particularly, include the following. Firstly, and perhaps most serious, is the formation of sulfates by oxidation of the sulfites formed in the scrubber. Presumably, such sulfates result from reaction of the oxygen contained in the flue gas with either sulfite or bisulfite or by disproportionation of the sulfite or bisulfite. In any case, the relatively insoluble sulfate builds up in any regenerative system, that is one where the additive is regenerated for reuse in the scrubber, thus creating operational difficulties as well as resulting in consumption of absorbent. To prevent such build-up, a portion of the absorbent must be discarded, thus causing a waste of useful absorbent. Secondly, the regeneration of the additive has always been a problem in terms of cost. Any regeneration procedure which is effective but cheaper than those now known will be an improvement. Thirdly and lastly, the concentration of the additive in aqueous absorbents of the foregoing type has generally been small. Yet, adequate contact between gas and additive requires a relatively large volume of aqueous absorbent. Such dilute aqueous solutions impose an undesirably low temperature of operation in the scrubber. Higher concentrations of salts in the aqueous absorbent result in lower water vapor pressure over the scrubbing solution and thus permit higher scrubbing temperatures; and hence less reheat is required before the scrubbed gas is released to the atmosphere.
The primary object of the present invention is to provide an improvement in the processes of the type described which alleviates these problems. Patents which may be considered pertinent to the improvement are as follows:
U. S. 1,795,121 Hansen Mar. 3, 1931 U. S. 2,729,543 Keller Jan. 3, 1956 U. S. 3,236,589 Reinhall et al. Feb. 22, 1966 U. S. 3,431,070 Keller Mar. 4, 1969 U. S. 3,523,755 McRae Aug. 11, 1970 U. S. 3,574,097 Urban Apr. 6, 1971 U. S. 3,644,087 Urban Feb. 22, 1972 U. S. 3,710,548 Coughlin Jan. 16, 1973 Br.Pat. 134,943 Nov. 17, 1919 Br.Pat. 292,186 June 11, 1928 Br.Pat. 459,418 Jan. 7, 1937 German 208,633 May 16, 1908